This invention relates generally to safety equipment for electrically controlled automated equipment. More particularly, the invention relates to equipment for monitoring the status of emergency stop equipment, such as safety gates, manually operated switches, safety interlocks and the like. Although the invention finds particular utility in manufacturing plants and on assembly lines having multiple work stations or where machines work in concert, the invention is equally useful in other monitoring applications.
In an assembly plant or manufacturing plant utilizing automated equipment, it is frequently advantageous to coordinate the operation of a plurality of pieces of automated equipment to work in concert with one another. In this fashion, a workpiece undergoing manufacture proceeds smoothly and efficiently from one work station to the next without inventory buildup or backlog. Each such work station may include a number of different pieces of automated or semi-automated equipment, such as welding stations, shuttles, stamping presses, turn-over devices, bending jigs, and the like. The time during which a workpiece spends at each station will, in general, depend on a number of factors, such as the particular manufacturing process being performed, the size, shape and other physical characteristics of the workpiece, and the moninal speed at which the human operator is working. Frequently considerable thought and effort is devoted to coordinating the times a workpiece spends at each work station in order to prevent unnecessary inventory buildup or backlogs.
Many modern day manufacturing plants which utilize automated equipment employ emergency stop equipment for stopping the automated equipment, or preventing it from starting, in the event of an emergency or to permit maintenance crews to effect repairs. In many applications, particularly those which employ a plurality of pieces of automated equipment working in concert, the emergency stop mechanism for each piece of automated equipment is integrated with other emergency mechanisms to provide an emergency stop system for the entire coordinated assembly line, or at least major portions thereof. In such integrated emergency stop systems a fault or emergency occurring at one work station causes the shut-down of all coordinated work stations until the fault is corrected and the system is reset.
In accordance with present day practices, the integrated emergency stop system usually employs a plurality of series connected, normally closed electrical current interruption devices, each providing a different safety function. The plurality of series connected devices are coupled to receive current from a source of electrical power and to conduct that current to the coil of a relay or other type of current sensing load. When any one of the series connected devices is opened, current flow through the relay coil or other sensor is interrupted, causing power to be interrupted from one or more of the pieces of automated equipment. In conventional practice, the series connected current interrupting devices may be implemented using a variety of different mechanisms, including push button switches for manual operation, safety gates equipped with jumpered terminals which break current flow when opened, electronically controlled or microprocessor controlled switches, and so forth. Such emergency stop systems are provided with test points at the nodes or interconnections between series connected current interruption devices In many applications, several groups of series connected current interruption devices are arranged as parallel branches or legs across the hot and common buses of an alternating current distribution system. Each branch or leg thus comprises a plurality of series connected current interruption devices which deliver logic current to a relay coil in series therewith. Each relay may provide a plurality of contacts or outputs which may be used to make or break current for powering the automated equipment on the assembly line. Frequently one pair of contacts on each branch relay are series connected with one another to make and break control current to a master relay (or groups of master relays) for interrupting power to the entire assembly line when a fault occurs. Such a feature is particularly useful where pieces of automated equipment work in concert with one another and must therefore be shut down whenever a fault occurs in one of the pieces of equipment.
When a fault occurs somewhere in a complex integrated system of the type described above, it is often quite difficult and time consuming to localize and identify which piece of equipment, safety interlock or switch has caused the system to shut down. By examining or metering the plurality of relay contacts a technician can often locate which branch has caused the shut down, although such inspection or metering does not reveal which device in that branch has caused the fault. In order to locate precisely which device has caused fault, the technician has heretofore been forced to visually inspect each of the current interrupting devices or to take voltage measurements at each of the test points along the branch until the open circuit is isolated and identified. In a complex control system, as found in many manufacturing plants and along many assembly lines, such procedures can cause costly down time. Even more difficult to identify is the intermittent fault, which may last long enough to trip the emergency stop relays, but which repairs or corrects itself before the technician can localize and identify it. Often times faulty components can produce a frustrating string of such intermittent faults before the device fails completely. There has heretofore been no economically way of quickly locating either the sustained fault or the intermittent fault.
The present invention solves the problem of identifying both the sustained fault and the intermittent fault quickly and easily. The invention is well suited for use in manufacturing plants and on assembly lines or wherever control circuits are used to control automated or semi-automated equipment. In accordance with the invention an apparatus for monitoring the activity states of a plurality of current altering devices in a circuit capable of conducting current from a source to a load is provided. The apparatus comprises a monitoring means electrically coupled to the current altering devices for providing a plurality of fault status signals. These fault status signals indicate the activity states of each of the current altering devices. Preferably, such monitoring occurs continuously. The invention further includes a means for providing a strobe signal when the current flow is altered or interrupted by one or more of the current altering devices. A means for storing the fault status signals in response to the strobe signal is provided which produces logic signals indicative of the stored fault status signals. A fault identifying means is responsive to the logic signals for identifying at least one of the devices which altered or interrupted the current flow and for providing an indication thereof. When one or more of the current flow altering devices causes a fault, alteration or break in the current flow, the activity statuses of all current altering devices are strobed into and latched in the storing means where the status signals remain latched until the system is manually reset. In this fashion, even intermittent faults can be readily localized and identified. The fault identifying means compares the stored fault status signals and identifies the current altering device closest to the source of control power supply which has caused the break in control current. The invention is configured to permit a plurality of such activity or conductivity state monitoring apparatuses to be cascaded together, in daisy-chain fashion. While the invention is well adapted for monitoring series connected current interrupting devices, the invention is equally useable for monitoring independently operating devices.
For a more complete understanding of the invention, its objects and advantages, reference may be had to the following specification and to the accompanying drawings.